[Circadian changes in physiological urinary excretion of glycosaminoglycans in healthy rats]

Particular chemical structure and original molecular arrangement make the glomerular basement membrane (GBM) to play a key role in the glomerular filtration, by restraining selectively the transglomerular pathway of high molecular weight charged macromolecules. The present study gives evidence, for the first time in the literature, of circadian changes in physiological urinary glycosaminoglycans (GAGs) excretion, main components of the GBM, in normal rats. Urine was collected every 4 hours during 6 consecutive spans in 45 Sprague-Dawley rats, housed in individual metabolism cages (8.00-12.00, 12.00-16.00, 16.00-20.00, 20.00-00.00, 00.00-04.00 and 04.00-08.00). GAGs, water, protein and electrolyte excretion were quantified in each sample. The analysis of these solute temporal changes was done by the cosinor method and shows that a circadian rhythm can be detected. These rhythms exhibit a significant increase between 20.00 and 8.00, with a peak between 00.00 and 04.00 and a minimum between 12.00 and 20.00. A precise estimation of the rhythmic characteristics permitted to detect the acrophase of each parameter nearly at the same time, respectively at 23.25 for diuresis, 23.33 for GAGs, 23.32 for proteins, 23.32 for sodium and 23.44 for potassium excretion. Such chronophysiological data suggest a tight correlation between urinary GAGs excretion and urinary water and electrolyte excretion in physiological conditions, in normal rats. Because of the great complexicity of the factors involved in the kidney the renal excretion regulation, the different origins of these temporal changes are largely discussed.